Generally described, a positive displacement pump delivers a fixed volume of liquid for each cycle of pump operation. The only factor that impacts the flow rate in an ideal positive displacement pump is pump speed. The flow characteristics of the overall system in which the pump operates should not impact the flow rate therethrough.
In practice, variations exist between the theoretical flow rate and the actual flow rate due primarily to influences from the volumetric efficiency of the pump, pump slippage (internal fluid bypass from the outlet to the inlet), system pressure, and fluid viscosity. Each individual pump could have different performance characteristics dependent on these and other variables.
Thus, there is a desire for a pump that can accommodate the different influences such as fluids of differing viscosities and volumetric efficiencies. Specifically, the pump system should accommodate different fluid characteristics and variations in the system itself.